Nuclease p1 produced by Penicillium citrinum was purified and characterized. Fermentation was carried out for 49 h for optimum production. The enzyme was purified to homogeneity according to SDS-PAGE by thermal deactivation, ultrafiltration, (NH4)(2)SO4 precipitation, phenyl sepharose chromatography, ion-exchange chromatography and gel filtration. This protocol improved 93-fold purification and 5.4% recovery of enzyme activity. The molecular mass of nuclease p1 from Penicillium citrinum was estimated to be in the range from 43 to 44 kDa. The enzyme showed an obligate requirement for Zn2+ for activity. K+ had a significant effect on enzyme activity. Mg2+ and Ca2+ had a slight positive effect on enzyme activity. While Cu2+, Co2+, Al3+, SDS and Triton X-100 were strong inhibitors of nuclease p1. The enzyme was completely stable in the pH range from 5 to 6.5 and had a pH optimum of 5.4. The optimum temperature of nuclease p1 was 69 degrees C and enzyme was stable up to 70 degrees C. K-m and V-max for the purified enzyme were 24.28 mg/ml and 0.36 mg/ml min, respectively, with RNA as substrate. (c) 2006 Elsevier Ltd. All rights reserved.